Glutamate dehydrogenase (GDH) and lysine-sensitive aspartokinase (AKIII) are inactivated in Escherichia coli cells starved for ammonia or glucose. Inactivation of GDH and AKIII is blocked in relA mutants during ammonia starvation, but occurs normally in cya (cAMP-deficient) and deg (degradation-deficient) mutants. Immunochemical assays indicate that both GDH and AKIII are degraded during inactivation. No fragments of either enzyme have been positively identified in immuno-precipitates obtained during degradation, but deg mutants do accumulate at least one lower molecular weight peptide that binds to an anti-GDH antibody column. Purified GDH is highly resistant to proteolysis by proteases such as trypsin, chymotrypsin, subtilisin, and thermolysin. When NADPH is present at 10-200 micronsM, GDH is rapidly degraded by these same proteases. Proteolysis in the presence of NADPH can be prevented by alpha-ketoglutarate, glutamate, and numerous other anionic metabolites. GDH is stable in crude extracts of E. coli prepared by several methods, with or without NADPH. GDH in cells permeabilized with toluene undergoes a slight (10-30%) and variable loss of activity which is dependent on NADPH.